The present invention relates to a system and method for demodulating a data packet in a communication system using automatic repeat request (“ARQ”) protocol, specifically an ARQ communication system that combines the energy of an initially transmitted data packet with the energy of a retransmitted version of the data packet. The inventive system and method measures the “usability” of each data packet if the data packet does not pass an error-checking procedure. If a data packet is determined to be “usable”, the energy of the data packet is stored and/or combined with the energy of a previously received version of the same data packet that has been determined to be “usable”. The combined data packet may then be combined with a subsequently received version of the same data packet and then subjected to the error-checking procedure. This sequence of steps may repeat until the combined data packet successfully completes the error-checking procedure.
As used herein, “packet” or “data packet” is meant to be interpreted, as known to those of skill in the art, as (a) a group of logical 1's and 0's that are used to transmit information, and/or (b) a “signal packet” which is a transmitted/received representation of at least one data packet that can be, for example, combined at the receiver according to the present invention.
In communication systems, transmitted data may be received with errors, i.e., the received data may differ from the transmitted data. The ARQ protocol is used to detect, but not correct, any errors in the transmission/reception of data. Typically, data packets incorporate error detection bits so that when a receiver processes a data packet, the error detection bits are checked to determine if the data packet was received correctly. If the packet was received correctly, an acknowledgement signal (“ACK”) is sent from the receiver to the transmitter which indicates to the transmitter that the packet was correctly received. If the packet was not received correctly, a negative acknowledgement signal (“NAK”) is sent from the receiver to the transmitter which indicates to the transmitter that the packet needs to be retransmitted. In some communication systems, the non-receipt of an ACK by the transmitter within a predetermined amount of time is interpreted the same as the receipt of a NAK, which implies that the transmitter will retransmit the data packet. It shall be understood that any reference to the transmission and/or reception of a NAK includes the situation where an ACK is not transmitted and/or received within a predetermined amount of time.
Certain prior art ARQ systems use a “selective” retransmission scheme, a “traceback” scheme, or a “stop-and-wait” scheme. In an ARQ system with selective retransmission, the transmitter will retransmit those packets for which a NAK (or the lack of an ACK) has been received. In an ARQ system with traceback, the transmitter will retransmit the packet for which a NAK (or the lack of an ACK) has been received and all subsequently-sent packets which were transmitted prior to receiving the NAK (or ACK lack). In an ARQ system employing a “stop-and-wait” scheme, a packet is sent by the transmitter and an ACK must be received by the transmitter before the next packet in the queue is sent.
In noisy communication channels, a large number of retransmissions may occur before data is correctly received. If each received data packet is processed independently at the receiver, the communication system is at the mercy of the noisy environment because the receiver's probability of correctly receiving a given transmitted data packet is the same as the probability of correctly receiving the previously-transmitted version of the data packet. If the receiver combines the energy of the multiple data packets that are received, the probability of correctly receiving the data packet is enhanced with the addition of the energy of each additional data packet received.
Typical prior art communication systems operating with an ARQ protocol may enhance their ability to successfully receive a data packet by combining the energy of previously-received packets with the energy of one or more retransmissions of those packets. This technique is especially useful during operation of the communication system when the energy from a single data packet is insufficient to allow for successful reception of the data packet. Successful receipt of a data packet means that the information contained within the data packet is retrieved from the data packet, i.e., the data packet is demodulated and successfully error checked. Prior art systems which combine the energies of multiple transmissions of a data packet combine the packet energies without regard to the “usability” of the content of the data packet which is typically a function of whether the receiver was synchronized with the transmitter when the data packet was transmitted/received.
Combining the energy of multiple transmissions of a data packet, specifically in an additive white Gaussian noise channel, dramatically improves the probability of receiving the data packet successfully. However, when a receiver's demodulator (e.g., a modem) loses synchronization (i.e., is “out-of-sync”) with the transmitter sending the data packet, the packet energy from the transmitted packet has no usable information content and is similar to pure noise from the detection point of view. If the energy of the out-of-sync data packet is added to the energy pool of previously-received transmissions of the same data packet, the out-of-sync data packet will pollute the energy pool thereby preventing successful reception of the data packet until the energy from enough subsequently-retransmitted in-sync data packets are added to the energy pool to overcome the deleterious effect of the out-of-sync data packet. The addition of the energy of an out-of-sync data packet to the energy pool necessitates multiple retransmissions of the data packet to overcome the energy of the out-of-sync data packet which significantly degrades the throughput of the communication system and may completely negate the advantages of combining the energy of retransmitted data packets to ensure successful reception. Prior art communication systems which automatically add the energies of multiple data packet transmissions without regard to the “usability” of the data packet are susceptible to significantly reduced throughput and performance degradation due to the addition of the energy of an out-of-sync packet to the energy pool.
Another problem faced by prior art communication systems which automatically add the energy of multiple transmissions of a data packet is the susceptibility to jamming signals. If a jamming signal is received by the receiver in the place of a retransmission of a data packet, the energy of the jamming signal is then added to the energy pool for the data packet. Since the energy of a jamming signal is typically much greater than the energy of a data packet, the energy pool may be insurmountably polluted and the communication system severely degraded.
Thus there is a need for a communication system using ARQ protocol which can determine whether to add the energy of a received data packet to the energy pool for that data packet and therefor overcome the limitations of the prior art systems and methods.
“The present invention overcomes the limitations of the prior art communication systems by including functionality at the receiver to determine if a received data packet is “usable”, e.g., whether the receiver is in sync with the transmitter when the data packet is received. If a data packet is determined to be usable, the energy of the data packet may be summed with the energy of previously-transmitted data packets in order to attempt to successfully receive the data packet, i.e., recover the information in the data packet. If a data packet is determined to be unusable, the energy of the data packet is not added to the energy pool.”
One embodiment of the present invention uses a viterbi decoder as part of the signal tracking loop at the receiver. As is known in the art, signal loss detectors can be made by comparing the minimum and maximum path metric of a viterbi decoder. The present invention uses a similar technique to determine whether to keep or discard the energy of a received packet. The minimum and maximum path metrics of a viterbi decoder, such as a short viterbi decoder, are used as a signal quality indicator to determine whether the energy of a received data packet is likely to make the energy pool for that data packet better or worse. If it is determined that the energy of the received data packet will make the energy pool better, the energy is added to the energy pool. On the other hand, if it is determined that the energy of the received data packet will make the energy pool worse, the energy is not added to the energy pool. If the determination is that the energy of the received data packet will make the energy pool worse, the receiver may attempt to decode the data packet by itself.
The present invention operates in a communication system with either selective ARQ protocol or ARQ protocol with traceback.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel system and method for avoiding degradation of an ARQ communication system which combines the energy of multiple data packets.
It is another object of the present invention to provide a novel system and method for determining whether to combine the energy of a received data packet with the energy pool of previously-received versions of the data packet.
It is yet another object of the present invention to provide a novel system and method for improving the throughput of a communication system by receiving a signal containing an encoded data packet in an ARQ-enabled communication system transmitted packetized data from a transmitter to a receiver.
It is still another object of the present invention to provide a novel system and method for using a short viterbi decoder to determine whether to add the energy of a received data packet to the energy pool of previously-received versions for the data packet.
It is a further object of the present invention to provide a novel system and method for using the minimum and maximum path metrics of a viterbi decoder as a signal quality indicator for determining whether to add the data packet energy to a running summation of data packet energy.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.